I was just watching STOked 78 and they mentioned the rule about the use of canon characters, namely: don't do it!

But I was wondering. What is the definition of a 'canon character'? I mean Spock, Kirk, Picard, Data and Janeway are obviously canon, but what about... a character like: Hoex (see: http://memory-alpha.org/wiki/Hoex). Is that allowed?

If a character has appeared in any show or movie, then they are a canon character. You may TALK about canon characters in your mission, and you may even quote them(as in, someone else is relaying their orders to you). You simply cannot SHOW them.